| Message ID | 1408453651.3746.41.camel@linaro1.home |
|---|---|
| State | New |
| Headers | show |
On Tue, Aug 19, 2014 at 02:07:31PM +0100, Jon Medhurst (Tixy) wrote: > If the device-tree specifies a max-memory-bandwidth property then > the CLCD driver uses that to calculate the bits-per-pixel supported, > however, it doesn't ensure that the result is a sane value, i.e. a > power of 2 and <= 32 as the rest of the code assumes. > > Acked-by: Pawel Moll <pawel.moll@arm.com> > Signed-off-by: Jon Medhurst <tixy@linaro.org> > --- > > This fixes code which is new in 3.17 (commit d10715be03) and so I assume > is a candidate for adding to a coming -rc ? Without the fix, people can > be left (as I was) with a blank non-functioning screen even if they > create a valid device-tree for the new driver functionality. > > drivers/video/fbdev/amba-clcd.c | 14 ++++++++++---- > 1 file changed, 10 insertions(+), 4 deletions(-) > > diff --git a/drivers/video/fbdev/amba-clcd.c b/drivers/video/fbdev/amba-clcd.c > index beadd3e..98b66b7 100644 > --- a/drivers/video/fbdev/amba-clcd.c > +++ b/drivers/video/fbdev/amba-clcd.c > @@ -24,6 +24,7 @@ > #include <linux/list.h> > #include <linux/amba/bus.h> > #include <linux/amba/clcd.h> > +#include <linux/bitops.h> > #include <linux/clk.h> > #include <linux/hardirq.h> > #include <linux/dma-mapping.h> > @@ -650,6 +651,7 @@ static int clcdfb_of_init_display(struct clcd_fb *fb) > { > struct device_node *endpoint; > int err; > + int bpp; > u32 max_bandwidth; > u32 tft_r0b0g0[3]; > > @@ -667,11 +669,15 @@ static int clcdfb_of_init_display(struct clcd_fb *fb) > > err = of_property_read_u32(fb->dev->dev.of_node, "max-memory-bandwidth", > &max_bandwidth); > - if (!err) > - fb->panel->bpp = 8 * max_bandwidth / (fb->panel->mode.xres * > + if (!err) { > + bpp = 8 * max_bandwidth / (fb->panel->mode.xres * > fb->panel->mode.yres * fb->panel->mode.refresh); This calculation is wrong in any case - this is the naïeve calculation which assumes that the bandwidth is: x * y * (bpp / 8) * refresh That isn't the maximum bandwidth, it's the average bandwidth across a full frame. If we're interested in limiting the maximum bandwidth, because the hardware can't cope with fetching the data above a certain rate, then we need a different method. We know the pixel rate. We know how many memory bits are fetched for each pixel. So: peak_bandwidth = pixel_clock * bpp / 8 Let's take 32bpp, 1024x768@60Hz, which has a pixel clock of 65MHz. Using the first, we get: 1024 * 768 * 4 * 60 = 180MiB/s. Using the second: 65 * 4 = 248MiB/s. To see why there's this discrepency, realise that there's 320 clocks where there's no pixel activity (so no memory fetches) per line, and 38 lines where there are no memory fetches. That's 320 * 768 + 1344 * 38 = 296832 pixel clocks where there's no memory fetches, out of a total of 1344 * 806 = 1083264 total pixel clocks in a frame. So, that's about 27% of the frame where the memory subsystem is inactive - which accounts for the difference in the above figure. So, if you are limiting the bandwidth of the mode due to the available bus bandwidth to the controller (which is implied by the property name), then you want to use the second equation to limit the peak bandwidth, and not the average bandwidth.
On Tue, 2014-08-19 at 15:40 +0100, Russell King - ARM Linux wrote: > On Tue, Aug 19, 2014 at 02:07:31PM +0100, Jon Medhurst (Tixy) wrote: > > If the device-tree specifies a max-memory-bandwidth property then > > the CLCD driver uses that to calculate the bits-per-pixel supported, > > however, it doesn't ensure that the result is a sane value, i.e. a > > power of 2 and <= 32 as the rest of the code assumes. > > > > Acked-by: Pawel Moll <pawel.moll@arm.com> > > Signed-off-by: Jon Medhurst <tixy@linaro.org> > > --- > > > > This fixes code which is new in 3.17 (commit d10715be03) and so I assume > > is a candidate for adding to a coming -rc ? Without the fix, people can > > be left (as I was) with a blank non-functioning screen even if they > > create a valid device-tree for the new driver functionality. > > > > drivers/video/fbdev/amba-clcd.c | 14 ++++++++++---- > > 1 file changed, 10 insertions(+), 4 deletions(-) > > > > diff --git a/drivers/video/fbdev/amba-clcd.c b/drivers/video/fbdev/amba-clcd.c > > index beadd3e..98b66b7 100644 > > --- a/drivers/video/fbdev/amba-clcd.c > > +++ b/drivers/video/fbdev/amba-clcd.c > > @@ -24,6 +24,7 @@ > > #include <linux/list.h> > > #include <linux/amba/bus.h> > > #include <linux/amba/clcd.h> > > +#include <linux/bitops.h> > > #include <linux/clk.h> > > #include <linux/hardirq.h> > > #include <linux/dma-mapping.h> > > @@ -650,6 +651,7 @@ static int clcdfb_of_init_display(struct clcd_fb *fb) > > { > > struct device_node *endpoint; > > int err; > > + int bpp; > > u32 max_bandwidth; > > u32 tft_r0b0g0[3]; > > > > @@ -667,11 +669,15 @@ static int clcdfb_of_init_display(struct clcd_fb *fb) > > > > err = of_property_read_u32(fb->dev->dev.of_node, "max-memory-bandwidth", > > &max_bandwidth); > > - if (!err) > > - fb->panel->bpp = 8 * max_bandwidth / (fb->panel->mode.xres * > > + if (!err) { > > + bpp = 8 * max_bandwidth / (fb->panel->mode.xres * > > fb->panel->mode.yres * fb->panel->mode.refresh); > > This calculation is wrong in any case - this is the naïeve calculation > which assumes that the bandwidth is: > > x * y * (bpp / 8) * refresh > > That isn't the maximum bandwidth, it's the average bandwidth across a > full frame. > > If we're interested in limiting the maximum bandwidth, because the > hardware can't cope with fetching the data above a certain rate, then > we need a different method. > > We know the pixel rate. We know how many memory bits are fetched for > each pixel. So: > > peak_bandwidth = pixel_clock * bpp / 8 That all sounds logical and reasonable to me, especially as the binding doc describes the property as "maximum bandwidth in bytes per second that the cell's memory interface can handle". I'll update the patch to also correct the calculation as you suggest.
diff --git a/drivers/video/fbdev/amba-clcd.c b/drivers/video/fbdev/amba-clcd.c index beadd3e..98b66b7 100644 --- a/drivers/video/fbdev/amba-clcd.c +++ b/drivers/video/fbdev/amba-clcd.c @@ -24,6 +24,7 @@ #include <linux/list.h> #include <linux/amba/bus.h> #include <linux/amba/clcd.h> +#include <linux/bitops.h> #include <linux/clk.h> #include <linux/hardirq.h> #include <linux/dma-mapping.h> @@ -650,6 +651,7 @@ static int clcdfb_of_init_display(struct clcd_fb *fb) { struct device_node *endpoint; int err; + int bpp; u32 max_bandwidth; u32 tft_r0b0g0[3]; @@ -667,11 +669,15 @@ static int clcdfb_of_init_display(struct clcd_fb *fb) err = of_property_read_u32(fb->dev->dev.of_node, "max-memory-bandwidth", &max_bandwidth); - if (!err) - fb->panel->bpp = 8 * max_bandwidth / (fb->panel->mode.xres * + if (!err) { + bpp = 8 * max_bandwidth / (fb->panel->mode.xres * fb->panel->mode.yres * fb->panel->mode.refresh); - else - fb->panel->bpp = 32; + bpp = rounddown_pow_of_two(bpp); + if (bpp > 32) + bpp = 32; + } else + bpp = 32; + fb->panel->bpp = bpp; #ifdef CONFIG_CPU_BIG_ENDIAN fb->panel->cntl |= CNTL_BEBO;